Women are reported to smoke more in the luteal phase of the menstrual cycle. It is plausible that increased metabolism of nicotine, secondary to the effects of higher estrogen and/or progesterone levels during the luteal compared with the follicular phase of the menstrual cycle could be the cause. The study will determine the effect of different phases of the menstrual cycle on: (1) the metabolism of nicotine and cotinine (2) the acute effects of nicotine on the CV system and (3) the stimulation by nicotine of ACTH release.